The concept of a site is to represent physical locations in a wide-area network.
Site definitions support the control of replication. Site knowledge is also used
for logon location, printer location, and other uses. Sites group domain
controllers that are physically located in well-connected areas.

Intrasite replication is automatically configured to replicate between
replication partners every 5 minutes. Because there is a maximum of 3 hops
between domain controllers, replication is completed within 15 minutes. A
service called KCC Knowledge Consistency Checker automatically determines the
replication topology. The replication topology specifies which DCs exchange
information (replication partners) such that there are no more that 3 hops
between any two servers.

Intersite replication is configurable and schedulable.

New York site

Los Angeles
site LA

çintrasite
replicationè

çintersite
replicationè

çintrasite
replicationè

Site information is used for

Controlling the replication topology

Scheduling replication

Locating services within a site

Sites
and Services Manager

Sites
are created with the Sites and Services Manager. Rather than designing some new
way of defining a site, the Active Directory uses the existing TCP/IP subnet
structure that is already defined for routers to find various sites. The Active
Directory sites do not need to follow the subnet structure exactly. Site subnet
masks can be adjusted to combine or divide the TCP/IP subnets. For example, a
site defined with the address 209.47.184.0 and mask 255.255.255.0 defines the
range of addresses 209.47.184.1 to 209.47.184.254 as being in the same site.

Each site is associated with a subnet object
which is defined by a IP network address and a subnet mask. All computers within the subnet
address range are considered to be at the same site. Site definitions normally
match the physical subnets, but they can be specified to combine or divide
physical subnets.

Each site connects to
other sites with a site link which has
configuration information that controls replication.

The site link configuration shown in the
following dialogs includes

Link cost

Replication frequency

Replication Schedule

The replication schedule is easily controlled
by modifying the matrix shown in the following dialog.

How does a client determine its site?

Administrators are not required to configure
the site for each client. Portable computers often change their site locations
and they need to automatically determine their new location without manual
administrative effort. This problem also exists and already has solutions in the
area of TCP/IP configuration. When portable computers change locations, they
need to have a new IP address assigned. This can be done manually, but is most
often done automatically by DHCP servers.

When a client computer starts, it will connect
to any domain controller in its domain. The domain controller looks at the
client IP address and compares it to the site definitions stored in its AD
configuration name context. If the domain controller is in the same site, it
will continue to provide services to the client. If the domain controller is in
a different site, it will redirect the client to a domain controller in that
site.

èI
need to connect to a DC, here is my IP address èçYou
are in my site so I will look after you ç

èI
need to connect to a DC, here is my IP addressèçYou
are in a different site, connect to a server in your siteç

The site location is remembered in the registry
in the key
HKLM\System\CurrentControlSet\Services\Netlogon\Parameters

Value DynamicSiteName records the last known
site location

Value SiteName specifies a site location
that will override the dynamic discovery